Wireless communication techniques are applied not only to the field of information communication but also to the field of logistics management and the like, and an IC tag for wireless communication (hereinafter, simply referred to as an “IC tag”) is widely known as a product that plays an important role in RFID (radio frequency identification) technology. Since an IC tag can be used in a wide range of applications for logistics management or low-cost information storage media, the IC tag is used in various environments.
The IC tag includes a chip that stores data such as an identification number and an antenna that is used for transmitting and receiving radio waves, and is significantly advantageous in that the IC tag can be realized as a small, thin, and light apparatus.
In order to sufficiently make use of such an advantage, it is preferable that there is no limitation on the position to which the IC tag is stuck, and that the IC tag is configured so as to be capable of performing communication regardless of the position or manner in which the IC tag is stuck.
However, the IC tag is designed to be used in the free space. In the case of using radio waves of a VHF band, a UHF band and a microwave band, since a general-purpose tag performs transmission/reception through a radio wave communication using a so-called dipole antenna, when a metal and the like exists in the vicinity of an antenna, communication properties of the antenna are degraded, so that a possible communication distance may be reduced.
In the case where a conductive material such as a metal exists in the vicinity of the antenna, when an electric current flows through the antenna, a reverse current is induced to the metal, so that input impedance of the antenna is significantly reduced by the induced current. Thus, impedance matching with an IC chip designed with respect to a free space is not achieved, so that a possible communication distance may be reduced.
In general, a dipole antenna, a monopole antenna and a loop antenna receive a radio wave with a specific frequency to generate a resonance current therein. When the resonance current flows through an IC chip, it is designed such that matching with chip impedance in the free space is achieved.
FIG. 25 is a cross-sectional view illustrating an electric field formed in the vicinity of a wireless IC tag 20 in the state in which the wireless IC tag 20 is arranged in the vicinity of a conductive member.
When a communication disturbing member 112 exists in the vicinity of an antenna element 111, an electric current I11 is generated from the other end portion 111b of the antenna element 111 to one end portion 111a thereof, and an electric current I12 is generated from one portion 112a of the communication disturbing member 112 to the other portion 12b thereof, so that a reverse current is generated in the antenna element 111 and the communication disturbing member 112.
Since an alternating current is applied by an IC 117, the generation state of the electric current in the direction illustrated in FIG. 25 and the generation state of the reverse current alternate. If a frequency becomes high, a state is reached, which is equivalent to a state as if an electric current I0 is generated between the one end portion 111a of the antenna element 111 and the one portion 112a of the communication disturbing member 112, and between the other end portion 111b of the antenna element 111 and the other portion 112b of the communication disturbing member 112. Thus, the one end portion 111a of the antenna element 111 is short-circuited from the one portion 112a of the communication disturbing member 112 in a high frequency manner, and the other end portion 111b of the antenna element 111 is short-circuited from the other portion 112b of the communication disturbing member 112 in a high frequency manner. When the above-described high-frequency short-circuit occurs, a closed circuit is formed by the antenna element 111 and the communication disturbing member 112, so that an electric current value is increased as compared with the case in which the communication disturbing member 112 does not exist in the vicinity of the antenna element 111. That is, as compared with the case in which the communication disturbing member 112 does not exist in the vicinity of the antenna element 111, impedance is reduced.
As described above, when a conductive material exists in the vicinity of the antenna or the chip, a reverse current relative to an electric current flowing through the antenna is induced to the surface of the conductive material, and a region of a high electric field and a region of a low electric field are connected to each other in a high frequency manner on the surface of the conductive material which faces the antenna, so that an electric path having a loop shape may be generated via the antenna and the conductive material. Since impedance is significantly reduced due to the generation of the electric path, matching with chip impedance is not achieved and an information signal cannot be transmitted. Thus, the possible communication distance may be reduced.
Further, as well as a metal, a paper, glass, resin, a liquid and the like may degrade the communication properties of the IC tag if they exist in the vicinity of the IC tag.
In the case of these materials, a resonance frequency of the antenna is changed according to a dielectric constant and permeability of these materials, and frequency shift occurs between a frequency of a radio wave used for a communication partner and the resonance frequency of the antenna, so that the possible communication distance may be reduced.
In addition, a product provided with the wireless IC tag is not limited to a single item, but may be provided in a plural number. Further, these may be aggregated, products having different sizes may coexist (the distance to a reader antenna is in a non-uniform state), or a case may occur in which a reading is necessary when a product is moving. When another RFID reader gate or metal material (radio wave reflector) exists adjacent to the product, a reading in a radio wave environment, which is different from an ideal environment such as a microwave darkroom, becomes necessary. In such a radio wave environment, radio interference may occur. Further, when wireless IC tags are aggregated or are in a state of movement, the probability that the wireless IC tags receive power necessary for starting or wireless communication is reduced, and a stable possible communication distance is difficult to ensure, so the possible communication distance may be reduced.
A wireless IC tag according to Japanese Unexamined Patent Application JP-A 2005-210676 includes an IC chip, a first antenna connected thereto, and a first spacer provided between the first antenna and a member provided with the IC chip. Further, the wireless IC tag includes a second antenna, and a second spacer provided between the first antenna and the second antenna.
With such a configuration, even when the IC tag is arranged in the vicinity of a metal, the communication distance can be increased.
According to JP-A 2005-210676, the second antenna serves as an auxiliary antenna, so that the communication distance is increased. However, since the auxiliary effect of the second antenna is actually small, the lengthening of the communication distance is not expected. Further, JP-A 2005-210676 discloses a wireless IC tag specially designed. In such a case, since the effect described therein is obtained using the special tag, there is no versatility such that communication improving effect can be exerted through the simple coupling of a commercialized wireless IC tag.
It is an object of the invention to provide a wireless communication improving sheet capable of improving a possible communication distance of an IC tag for wireless communication, a wireless communication IC tag, an information transmitting medium, and a wireless communication system.
It is another object of the invention to provide a sheet capable of improving wireless communication properties by simply tackinessly or adhesively applying a wireless IC tag.
The invention provides a wireless communication improving sheet for improving wireless communication properties of a wireless IC tag by arranging the wireless IC tag, comprising:
a first spacer having an arrangement surface for arranging the wireless IC tag without connecting the wireless IC tag with a wire;
an auxiliary antenna provided on a surface opposite to the arrangement surface of the first spacer; and
a second spacer provided on an opposite side to the first spacer with the auxiliary antenna interposed therebetween, they being laminated one on top of another,
the auxiliary antenna being formed with a hole or notch.
Further, in the invention, it is preferable that the auxiliary antenna includes a single or a plurality of conductor parts, and at least one of the conductor parts serves as a resonator which resonates with respect to an electromagnetic wave used for wireless communication.
Further, in the invention, it is preferable that the auxiliary antenna includes a plurality of conductor parts divided in a planar direction or a lamination direction, and at least one of the conductor parts serves as a resonator which resonates with respect to the electromagnetic wave used for wireless communication.
Further, in the invention, it is preferable that an adjusting unit formed with a hole or notch is provided for adjusting a resonance frequency of the wireless IC tag.
Further, in the invention, it is preferable that the adjusting unit is made of at least one material selected from the group consisting of a dielectric material and a magnetic material.
Further, in the invention, it is preferable that a wireless communication improving sheet further comprises a rear surface conductor layer provided on a side opposite to the auxiliary antenna of the second spacer.
Further, in the invention, it is preferable that the rear surface conductor layer is equal to or larger than the conductor part provided in the auxiliary antenna.
Further, in the invention, it is preferable that at least a part of the rear surface conductor layer protrudes in a resonance length direction of the conductor part provided in the auxiliary antenna.
Further, in the invention, it is preferable that the hole or notch is configured to face at least an IC chip or a reactance loading part provided in the wireless IC tag when the wireless IC tag is arranged.
Further, in the invention, it is preferable that the hole or notch is provided such that the auxiliary antenna resonates with respect to the electromagnetic wave used for wireless communication.
Further, in the invention, it is preferable that at least one of the first spacer and the second spacer is made of a foamed member.
Further, in the invention, it is preferable that a part or whole of an outer surface is coated with a dielectric material.
Further, in the invention, it is preferable that at least one of the arrangement surface and the surface opposite to the arrangement surface has adhesion or bonding properties.
Further, the invention provides a wireless communication IC tag obtained by arranging a wireless IC tag on the arrangement surface of the wireless communication improving sheet mentioned above, or by coupling an IC chip to the wireless communication improving sheet mentioned above.
Further, the invention provides a wireless communication IC tag obtained by combining a wireless communication IC tag whose coil antenna performing the wireless communication through electromagnetic induction improves communication properties by using a magnetic material, with the above-described wireless communication IC tag.
Further, in the wireless communication IC tag of the invention, a part or whole of an outer surface of the wireless IC tag is coated with a dielectric material.
Further, the invention provides an information transmitting medium comprising the wireless communication improving sheet or the wireless communication IC tag incorporated therein.
Further, the invention provides a wireless communication system using the wireless communication IC tag or the information transmitting medium.